Postural hypotension is the occurrence of low (systolic) blood pressure when sta

Question

Postural hypotension is the occurrence of low (systolic) blood pressure when standing up too quickly from a reclined position, causing fainting or lightheadedness. For most people, a systolic pressure less than 90 mm of Hg is considered low.

If the blood pressure in your brain is 120 mm when you are lying down, what would it be when you stand up? Assume that your brain is 40 cm from your heart and that = 1100 kg/m3 for your blood. Note: Normally, your blood vessels constrict and expand to keep your brain blood pressure stable when you change your posture.
=_____________mm of Hg

Explanation / Answer

The pressure differential between the brain and the heart upon standing up will be 120 - *g*h, where h = distance from the brain to the heart.

*g*h = 1100 kg/m^3 * 9.8 m/s^2 * 0.4 m = 4312 Pa

Converting Pa to mmHg:

4312 Pa x 760 mmHg/1.01325 x 10^5 Pa = 32.34 mmHg

So the pressure in the brain is now 120 - 32.34 = 87.66 mmHg (hypotension)

This value makes sense based on the physiological information in the problem.

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